In a colligative properties experiment, a lab group prepares four solutions and one pure solvent sample. what is the purpose of the pure solvent sample ?

The pure solvent sample will serve as the control for the experiment. 
The properties of a solution are different from the properties of the pure substance or the pure solvents. For instance, if water and salt are mixed together to form salt solution, the properties of the water and that of the salt are different from that of the salt solution. Colligative properties refers to the properties of solutions which depend on the ratio of the number of solute particles to the number of solvent molecules in a solution;  the properties do not depend on the chemical species that are present.